There are known wetland type sewage treatment plants with ground and root filter and horizontal or vertical flow of sewage, where the filter bed is composed of layers of gravel and sand. In sewage treatment plants of this type the top filter layer is planted with common reed or other hydrophilic vegetables. An oxygenic zone is created around the vegetable roots in result of oxygen transport to the roots through the shoots. Outside the oxygenic zone created in direct vicinity of the roots, there is an anoxic zone where anaerobic microorganisms are growing. The existence of the oxygenic zone and the anoxic zone results in increasing the diversity of microorganisms taking part in decomposition of sewage constituents, including oxidation of organic substance, nitrification, denitrification, and binding of phosphorus. The size of a ground and root-based sewage treatment plant depends on the quantity of inflowing sewage. Too small amount of sewage may result in withering of vegetables. Too big amount of sewage, as well as flooding with rainfall, may result in insufficient treatment of sewage and clogging of filter layers. Besides, the filter bed structure should provide adequate conditions for growth of microorganisms.
The patent description PL 198680 presents a biological sewage treatment plant with an exposed reservoir equipped with a biological filter bed with vertical flow of sewage. The filter bed consists of a lower layer of gravel, an intermediate layer of sand, and an organic top layer planted with swampy vegetation. The initially treated sewage is being collected by a drainage located at the bottom of the exposed reservoir, and directed gravitationally to a denitrification pond located below. The treated post-sewage water is directed to an inspection well. A disadvantage of this type of sewage treatment plant is the relatively large area of land, the reduced efficiency in winter season, and the gradual reduction of sorption ability, especially of the top layer.
The patent description U.S. Pat. No. 4,415,450 presents a method of sewage treatment using microorganisms and hydrophilic vegetation, where the sewage is subjected to initial treatment in anoxic conditions in a rotting tank for a period of 6 to 24 hours, and the effluent is directed to a filter bed with ascending flow. Thus the sewage is subjected to the action of microorganisms located in the lower part of the filter bed in the anoxic zone, and then to decomposition in the oxygenic zone. In the example of a sewage treatment plant presented in this solution, the lower layer of the filter bed contained rock aggregate of size 7.5 to 15 cm, the middle layer contained aggregate of size 2.5-7.5 cm, and the top layer contained gravel. Such structure of the bed results in shortening the time of sewage retention in the bed, irregular distribution of sewage in the bed, washing out of microorganisms, and flooding of the bed with precipitation.